A structure which forms a metallic reflecting layer as an optical reflecting layer between a substrate and an active layer composed of an MQW (Multi-Quantum Well) layer is proposed in order to perform the high brightness of an LED (Light Emitting Diode). As a method of forming such a metallic reflecting layer, it is disclosed about the wafer bonding technology of a substrate of a light emitting diode layer (Refer to Patent Literature 1 and Patent Literature 2, for example).
In Patent Literature 1 and Patent Literature 2, the purpose is to provide a fabrication method of a light emitting diode which can fabricate a light emitting diode having a desired mechanical characteristic and optical transparency, and can make a minimum specific electrical resistivity of boundary surface between a transparent layer and a growth layer; and it is characterized by fabricating the light emitting diode by removing a temporary growth substrate after growing up a light emitting diode layer one after another on the temporary growth substrate and forming a light emitting diode structure having a relatively thin layer, and wafer-bonding a conductive and optical transparent substrate on the light emitting diode layer which becomes a buffer layer of lower layer on the position instead of the temporary growth substrate. In Patent Literature 1 and Patent Literature 2, transparent materials, such as GaP and sapphire, are applied to the substrate used for the wafer bonding.
FIG. 1 to FIG. 3 show schematic cross-section structures of a conventional semiconductor light emitting device formed by the wafer bonding technology.
For example, as shown in FIG. 1, a conventional semiconductor light emitting device includes: an Au—Sn alloy layer 14 disposed on a GaAs substrate 15; a barrier metal layer 13 disposed on the Au—Sn alloy layer 14; a p type cladding layer 10 disposed on the barrier metal layer 13; an MQW layer 9 disposed on the p type cladding layer 10; an n type cladding layer 8 disposed on the MQW layer 9; and a window layer 7 disposed on the n type cladding layer 8.
In the conventional semiconductor light emitting device shown in FIG. 1, the metal used for the wafer bonding is Au—Sn alloy. As for the Au—Sn alloy, since the melting point is low, the Au—Sn alloy at the side of an epitaxial growth layer composing an LED in low temperature, and the Au—Sn alloy at the side of the GaAs substrate 15 can be melted and bonded.
However, since the thermal diffusion of Sn occurs when using the Au—Sn alloy layer 14, in order to prevent the diffusion of Sn, as shown in FIG. 1, it is necessary to insert the barrier metal layer 13. Moreover, there is a problem that the Au—Sn alloy layer 14 has a low optical reflection factor.
For example, as shown in FIG. 2, another conventional semiconductor light emitting device includes: a metallic reflecting layer 16 disposed on a GaAs substrate 15; a p type cladding layer 10 disposed on the metallic reflecting layer 16; an MQW layer 9 disposed on the p type cladding layer 10; an n type cladding layer 8 disposed on the MQW layer 9; and a window layer 7 disposed on the n type cladding layer 8. In the conventional semiconductor light emitting device shown in FIG. 2, there is a problem that light cannot be efficiently reflected from the metallic reflecting layer 16 fabricated by bonding the GaAs substrate 15 since the optical absorption occurs in the interface between metal and a semiconductor. That is, there is a problem that the optical absorption occurs in the interface between the p type cladding layer 10 and the metallic reflecting layer 16.
In order to perform high brightness of the semiconductor LED (Light Emitting Device), there is also a method of inserting a DBR (Distributed Bragg Reflector) layer between the GaAs substrate and the active layer (MQW) as an optical reflecting layer. The LED of the structure which does not insert the DBR becomes dark since the light which emitted from the MQW layer is absorbed by the GaAs substrate. Therefore, in order to perform the high brightness of the LED using the GaAs substrate, the DBR is used as the optical reflecting layer, for example.
That is, as shown in FIG. 3, another conventional semiconductor light emitting device includes: a DBR layer 19 disposed on a GaAs substrate 15; a p type cladding layer 10 disposed on the DBR layer 19; an MQW layer 9 disposed on the p type cladding layer 10; an n type cladding layer 8 disposed on the MQW layer 9; and a window layer 7 disposed on the n type cladding layer 8. In the conventional semiconductor light emitting device shown in FIG. 3, the DBR layer 19 is used as an optical reflecting layer between the GaAs substrate 15 and the MQW layer 9. However, there is a problem that the DBR layer 19 reflects only an incident light from a certain one way, the DBR does not reflect light if an incident angle changes, and the DBR layer 19 does not reflects an incident light from other angle and then the incident light passes through. Therefore, there is a problem that the passed through light is absorbed in the GaAs substrate 15 and the light emitting brightness of the semiconductor LED (Light Emitting Device) is reduced.
The conventional semiconductor light emitting device formed by the wafer bonding technology needs to insert the barrier metal layer, in order to prevent the thermal diffusion of Sn, when using the Au—Sn alloy layer as a metal used for the wafer bonding. Moreover, the Au—Sn alloy layer has a low optical reflection factor.
Moreover, even if the metallic reflecting layer is formed by bonding the substrate, the optical absorption occurs in the interface between the metal and the semiconductor, and then the light cannot be reflected efficiently.
Moreover, as mentioned above, when the DBR layer is used as the reflecting layer, the DBR layer reflects only an incident light from a certain one way, the DBR layer does not reflect and passes through the incident light if an incident angle changes, and the incident light is absorbed by the GaAs substrate, thereby the light emitting brightness of LED is reduced.
Furthermore, there is a problem that the conventional semiconductor light emitting device is easy to be removed if high temperature is applied since the conventional semiconductor light emitting device formed by the wafer bonding technology has the difference in a coefficient of thermal expansion and a problem of adhesion when bonding a semiconductor substrate, an insulating film, and a metal layer.
Furthermore, it is disclosed also about a semiconductor light emitting device bonds a layered structure and a semiconductor substrate of the semiconductor light emitting device by using an adhesive agent instead of the wafer bonding technology, and a fabricating method for such the semiconductor light emitting device (for example, refer to Patent Literature 3).    Patent Literature 1: Japanese Patent Application Laying-Open Publication No. H06-302857    Patent Literature 2: U.S. Pat. No. 5,376,580    Patent Literature 3: Japanese Patent Application Laying-Open Publication No. 2005-223207